A method for detecting an intersystem fault in the multi-circuit transmission line which comprises two lines of different voltages is described in Cigre Conference paper 34-206, Paris 1990, by H Becker, J Nilges and H Dirtrich entitled "Distance protection of double-circuit lines with different rated voltages during intersystem faults". The method is based on the distance protection principle.
To be able to selectively detect a short circuit according to the above-mentioned principle, it is necessary, when a short circuit occurs, to have access to correct values of currents and voltages at the measuring station. With the aid of the measured values at the instance of the fault, the circuit impedance, which corresponds to the positive-sequence impedance from the measuring station to the fault location, can be calculated. The method uses two current compensations which comprise the zero-sequence currents in the two lines and the corresponding voltages, which must be available at the same time. To detect an intersystem fault according to this method, it is thus assumed that the two lines can communicate with each other to be able to transmit measured fault quantities between the lines. This is, per se, a source of problems and it would therefore be a great advantage to have access to a method which is able to detect intersystem faults without having to communicate between the different lines. The present invention permits such a solution.